It is often desirable to visually inspect items, such as beverage cans, as they speed along a conveyor belt toward processing equipment. Automatic inspection systems typically use a stationary camera to acquire an image of each item and a processor to analyze the image for defects. Today's processing equipment may require the conveyor to move the items at speeds at or above 1800 items per minute. If a constant light source illuminates the items, the images are often blurred and thus unsuitable for the processor to inspect. That is, the image-acquiring speed of the camera is often insufficient to acquire sharp images in ambient light.
In high-speed visual inspection systems, the processor fires a strobe to briefly illuminate the item while it is within the camera's field of view. Such brief illumination, typically in the order of 50 microseconds, enables the camera to acquire an image sharp enough for inspection.
For controlling the timing of the strobe firing, a position sensor generates a signal as each item is a predetermined distance from the camera's field of view. After each signal, a controller waits for a "delay time" and then fires the strobe. In some inspection systems, the delay time is either manually calibrated or is set equal to the quotient of the predetermined distance divided by the expected conveyor speed. In other systems, a speed sensor measures the conveyor speed, and a processor calculates the delay equal to the quotient of the predetermined distance divided by a measured conveyor speed.